1. Field of Invention
The present invention relates to a semiconductor structure. More particularly, the present invention relates to the lower electrode structure of a capacitor that can withstand the flushing force produced by a cleaning agent.
2. Description of Related Art
Capacitor is the data storage section of a dynamic random access memory (DRAM). Data value of each memory cell is recorded as the amount of charges carried by the capacitor. Due to the miniaturization of memory cells, each cell is very small. To increase the capacitance of a capacitor and to reduce data errors and the number of memory refreshes, height of the lower electrode is frequently raised to increase the effective surface area of a capacitor.
FIG. 1 is a schematic cross-sectional view of the lower electrode of a conventional capacitor. To fabricate a conventional capacitor, a plurality of metal-oxide-semiconductor MOS transistors is formed over a substrate 100. Each MOS transistor includes a gate electrode 102 above the substrate 100, spacers 104 on the sidewalls of the gate electrode 102 and source/drain regions 106 in the substrate 100 on each side of the gate electrode 102. A dielectric layer 108 is formed over the substrate 100 covering the MOS transistor. A bit line 110 is formed in the dielectric layer 108. The bit line 110 is electrically connected to one of the source/drain terminals 106. Another dielectric layer 112 is formed over the substrate 100 covering the bit line 110. A plurality of lower electrodes 114 that pass through the dielectric layers 108 and 112 is formed with each lower electrode 114 electrically connected to a source/drain terminal 106.
FIG. 2 is a top view showing a plurality of conventional capacitor lower electrodes on a substrate. As shown in FIG. 2, a conventional lower electrode 114 has a rectangular shape. Before carrying out other processing treatment such as the growth of hemispherical silicon grains over the electrode for increasing effective surface area or the deposition of a capacitor dielectric layer, RCA cleaning solution is used to clean the surface. The RCA solution contains de-ionized water, sulfuric acid (H2SO4) and hydrogen peroxide (H2O2). The cleaning process includes repeated flushing of the lower electrodes 114 with de-ionized water, sulfuric acid solution and hydrogen peroxide solution.
However, as size of each memory cell is reduced, height of the capacitor lower electrode 114 must increase correspondingly to compensate for the shrinkage in surface area so that sufficient charge storage capacity is still present. Yet, the increase in height makes the lower electrode 114 more vulnerable to tearing when RCA solution is used to clean the surface. FIG. 3 is a top view showing the array of capacitor lower electrodes-after flushing with a cleaning solution. As shown in FIG. 3, a few of the lower electrodes 114 are flushed away from their original positions into new positions 114a. Due to the flushing damages, production yield of the process is lowered.
Accordingly, one object of the present invention is to provide a method of forming the lower electrode of a capacitor capable of withstanding the flushing force produced by a cleaning agent. A lower electrode having a rectangular profile when viewed from the top is provided. The lower electrode is bounded by a pair of ends and a pair of sides. The ends and the sides are linked together. The ends have a wedge shape. The sides have edges that cave in towards the center, thereby forming a recess region between the sides. A flushing operation is carried out using a cleaning solution. The cleaning solution flows from one end of the electrode to the other end along the sides.
This invention also provides an alternative method of forming the lower electrode of a capacitor capable of withstanding the flushing force produced by a cleaning agent. The lower electrode is a rectangular prism and has an hourglass shape when viewed from the top. The two sides of the lower electrode are edges that cave in towards the center. A recess region is defined between the two sides. A flushing operation is carried out using a cleaning solution. The cleaning solution flows from one end of the lower electrode to the other along the two sides of the lower electrode.
The step of flushing the lower electrodes can be carried out before other processing treatment such as the growth of hemispherical silicon grains over the electrode for increasing effective surface area or the deposition of a capacitor dielectric layer. The cleaning process includes repeated flushing of the lower electrodes with solutions such as de-ionized water, sulfuric acid solution and hydrogen peroxide solution.
The capacitor lower electrode structure of this invention has an hourglass profile and hence a more streamline body. According to fluid dynamics, resistant towards fluid motion is smaller. Consequently, the cleaning solution rushing past the lower electrode will cause less shearing and hence will result in the least damage. Furthermore, surface area of the lower electrode is also more than a conventional lower electrode, thereby increasing capacitance of the capacitor.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.